Conventionally, a radio communication system performs various kinds of system control to carry out communications with high quality and high efficiency. These kinds of control include, for example, transmit power control, communication channel control, cell changeover control, etc., which improves communication quality and promotes power saving as well.
Furthermore, a study on an adaptive communication system, which adaptively switches between modulation systems or between coding systems according to the communication quality of a radio communication link is also underway in recent years. For example, “Mobile Communication” (written and edited by Shuichi Sasaoka, Ohmsha, Ltd. P. 103 to 126) discloses a communication system using adaptive modulation, which is an example of an adaptive communication system. Communication quality on a communication link is often measured and used as information for making a decision on changeover between modulation systems or coding systems.
As indices of communication quality, a Bit Error Rate (BER), reception power and Carrier to Noise Ratio (CNR), etc., are often used. There is a method of estimating a bit error rate out of these indices and using it as an index of communication quality, whereby a known data string such as pseudonoise sequence is inserted in a transmission data string, this known data string is compared with a received data string and different data pieces are counted to thereby calculate the bit error rate.
There is another method whereby a transmission data string is subjected to error correcting coding, subjected to forward error correction at the time of reception and then recoded, this recoded data string is compared with the received signal string and different data pieces are counted to calculate the bit error rate. Furthermore, as disclosed, for example, in the Unexamined Japanese Patent Publication No. HEI 8-102727, a method of calculating a signal vector dispersion value and calculating a bit error rate from this dispersion value is also known.
With reference to FIG. 1, a configuration of a bit error rate calculation apparatus 1 will be explained as an example of a conventional apparatus for measuring communication quality below. In a communication system in which this conventional bit error rate calculation apparatus 1 is used, suppose data is transmitted with a predetermined data string inserted in a predetermined section of a transmission burst. For example, suppose a specific string of a pseudonoise sequence is inserted in the center of the burst. An orthogonal demodulation section 2 carries out orthogonal demodulation and synchronization processing on a received signal and outputs an orthogonal IQ vector string for each received symbol.
A bit decision section 3 carries out a bit decision using the orthogonal IQ vector string input and outputs the resulting received data. A known data section extraction section 4 extracts data corresponding to the section of the known data string inserted in the above-described burst from the received data string input and outputs the extracted data.
A bit error rate calculation section 6 compares the data string extracted by the known data section extraction section 4 with the data string stored in a known data storage section 5. When the received data contains some errors, the comparison results at the error locations show differences. Thus, by counting the number of bits with different comparison results for a predetermined period of time and calculating its ratio to the total number of bits compared, it is possible to calculate the bit error rate of the received signal statistically.
However, in a situation in which the bit error rate is low, calculating a statistically reliable bit error rate requires a sufficient total number of bits compared. As a result, it takes a long time to calculate the bit error rate.
For example, suppose a system that adaptively switches between two modulation systems of a QPSK modulation system and 16-value QAM modulation system according to the communication quality of a communication link as an example of the aforementioned adaptive communication system. Since the QPSK and 16-value QAM have different distances between signal points during modulation, these modulation systems differ in reception performance as shown in FIG. 2 and when data is received with the same reception power, it is generally known that QPSK has a lower bit error rate.
As information for making a decision in changeover between these two modulation systems, assume that modulation changeover between modulation systems is controlled using the result of bit error rate calculation using the bit error rate calculation apparatus 1 shown in FIG. 1 in such a way that the bit error rate does not exceed 1.0E-3. First, when the modulation system is changed from 16-value QAM to QPSK, it is possible to perform control in such a way that while monitoring a BER estimation result during 16-value QAM reception, the modulation system is changed to QPSK when this bit error rate exceeds an allowable value (e.g., 5.0E-4).
When the modulation system is changed from QPSK to 16-value QAM, it is likewise necessary to decide the changeover during a QPSK communication. For example, when Carrier to Noise Ratio (CNR) exceeds 17 dB during QPSK reception, the bit error rate of 16-value QAM also falls below 1.0E-3 as shown in FIG. 2, and therefore the changeover to 16-value QAM is decided.
FIG. 3 is an example of a distribution characteristic of an orthogonal IQ vector string for each received symbol obtained when QPSK reception demodulation is performed at a Carrier to Noise Ratio of 17 dB. Though signal points are dispersed due to influences of noise, there is almost no dispersion beyond the I and Q axes, and therefore bit errors with QPSK-occur only with a frequency equal to or lower than 1.0E-6. It takes an enormous number of reception bit samples and time for QPSK to check a bit error rate as small as 1.0E-6, and checking such a bit rate is unrealistic.
Thus, when the modulation system is changed from a modulation system such as QPSK with a relatively low bit error rate to a modulation system such as 16-value QAM with a relatively high bit error rate, there is a difficult problem in performing speedy changeover without increasing transmission errors involved in the changeover.